Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - Technical Issues D:OS without STEAM (Germany)

I think you should be safer with the GOG version for now.
Can't say how Gog galaxy will work, but for now they provide an installer you can download directly.

The Steam version is in principle DRM-free - you still need Steam to install it, and it will install updates through Steam, but you don't need it to run - I myself have made a link on the desktop with the exe in the D:OS directory and this completely bypasses Steam (as in, it's not running, I could probably delete Steam and move the D:OS directory somewhere else).
However, I'd guess that if there is a hint on the DVD cover that it needs Steam, the DVD version will still try to launch Steam at least once during the installation.